Humidity sensors comprising alkalimetal oxide,divanadium pentoxide and silicon

ABSTRACT

A humidity sensor having a negative relative humidity coefficient of resistivity comprises 0.10 to 12.00 mole percent of alkali metal oxide, 0.05 to 10.00 mole percent of divanadium pentoxide and the balance substantially all silicon.

United States Patent m Sumi et al.

51March 20, 1973 [541 HUMIDITY SENSORS COMPRISING ALKALI METAL OXIDE,DIVANADIUM PENTOXIDE AND SILICON [75] Inventors: Kiyoshi Sumi; OsamuAsakura,

both of Kyoto,Japan [73 AssigneeTShinyei Kaislia, lkuta-ku, Kobe [22]Filed: Aug. 16, 1971 [21] Appl. No.: 172,055

[52] US. Cl ..252/408, 338/35, 117/169 R, 117/169 A, 117/221, 117/222,117/223,

[51] Int. Cl. ..I-I0lc 13/00, GOln 31/06 [58] Field of Search ..338/35;252/408; 117/169 R, 117/169 A; 252/506, 507, 508, 509

Primary ExaminerGeorge F. Lesmes Assistant Examiner-Roland E. Martin,Jr. Attorney-Larson, Taylor & Hinds [5 7] ABSTRACT A humidity sensorhaving a negative relative humidity coefficient of resistivity comprises0.10 to 12.00 mole percent of alkali metal oxide, 0.05 to 10.00 molepercent of divanadium pentoxide and the balance sub stantially allsilicon.

10 Claims, 4 Drawing Figures HUMIDITY SENSORS COMPRISING ALKALI METALOXIDE, DIVANADIUM PENTOXIDE AND SILICON The present invention relatesgenerally to humidity sensors, more particularly to humidity sensorshaving a negative relative-humidity coefficient and it is also concernedwith a novel method of making these sensors.

Various humidity sensors have heretofore been known whereby changes inrelative humidity in an atmosphere are detected in terms of changes inelectrical resistivity (hereinafter referred to merely as resistivity).These humidity sensors have come into extensive use in recent years forhumidity measurement and control devices.

With the humidity sensors to be used for this type of devices, it isrequired that the resistivity vary regularly with the changes inhumidity over the entire range of the relative humidity.

Examples of the humidity sensors already known are a sensor prepared bycoating a high molecular weight substance serving as an insulator with athin film of a hygroscopic salt such as lithium chloride and a sensorpreparedby impregnatinga porous synthetic resin with a hygroscopic saltsuch as lithium chloride. In the case of these humidity sensors, thesalt dries or wets in corresponding relation to the changes in therelative humidity of the atmosphere and, by virtue of the flow ofcurrent, the changes in the amount of moisture taken up is detected interms of the changes in the resistivity of the humidity sensor. However,the above-mentioned hygroscopic salt, because of its propertiesdissolves in a highly humid atmosphere upon absorption of a large amountof moisture and becomes no longer serviceable as a resistive material,so that that the humidity sensors including such salt are subject to alarge limitation in that they can be used only under restricted humidityconditions and have the drawback that the resistivity varies greatlywith the changes in temperature.

Another humidity sensor is also known which is made by reacting anorganic terpene compound with ozone gas and causing the resulting fumingsubstance to be adsorbed to the single crystal of oxidized anthracene.The product thus obtained is used as a humidity sensor as it is. Sincethis sensor is inherently very high in resistivity, it is applicableonly to special devices which incorporate some other electrical elementshaving electrical characteristics corresponding to the high resistivityof the sensor. Moreover, the sensor has an inherent drawback that it islow in the reproductivity of the humidity changes in terms of thechanges in resistivity, namely it fails to exhibit accurate performanceas a humidity sensor.

Further in another sensor where magnetite is used as a resistivematerial, a colloidal liquid containing particles, for instance, with aparticle size of about 100 A is prepared and the liquid is then appliedor sprayed onto an insulating base plate to form a coating, sincemagnetite loses the humidity-resistance characteristics when fired tomore than 150 C. However, the coating of the humidity sensor obtained ispoor in mechanical strength, easy to peel off and becomes unusable oncepeeling takes place.

Accordingly, a primary object of this invention is to provide a humiditysensor which is capable of accurately detecting the changes in humidityin terms of the changes in resistivity over the entire range of therelative humidity.

Another object of this invention is to provide a humidity sensor whichis free of deterioration in respect of the humidity-resistivitycharacteristics thereof even when used repeatedly in a very humidatmosphere.

Another object of this invention is to provide a humidity sensor whichwill undergo hardly any change in resistivity when subjected to varyingtemperatures.

Another object of this invention is to provide a humidity sensor havingresistivity in a practical range and which therefore does not requirethe use of a special device including some other special electricalelements.

. Another object of this invention is to provide a humidity sensorhaving excellent mechanical strength.

Still another object of this invention is to provide a method formanufacturing a humidity sensor having the above characteristics.

These and other objects of this invention will become more apparent fromthe following description.

In principle, the present invention provides a humidity sensor having anegative relative humidity coefficient of resistivity which comprises0.10 to 12.00 mole percent of alkali metal oxide, 0.05 to 10.00 molepercent of divanadium pentoxide and the balance substantially allsilicon.

It has been found that the humidity sensor of this invention is capableof accurately detecting relative humidity since the resistivity thereofvaries regularly in the exponential functional relation to the change inhumidity over the entire relative humidity range; that repeated use in avery humid atmosphere does not impair the humidity-resistivitycharacteristics thereof; that the changes in temperature do not produceany substantial change in resistivity; that the sensor can be used forusual humidity measurement and in control devices since the changes inresistivity over the range of relative humidity of 0 to percent are in arelative low range of about 10fl-cm to IOQ-cm; and that the sensor hasexcellent mechanical strength.

With the humidity sensor of this invention, the amounts of the alkalimetal oxide and divanadium pentoxide are critical in achieving theeffects described above.

In the first place, it is required that the alkali metal oxide contentbe 0.10 to 12.00 mole percent. If it is lower 'than.0.10 mole percent,an exponential functional relation will not be obtained between therelative humidity and the .resistivity, while if it is in excess of12.00 mole percent, the resistivity may be in an exponential functionalrelation to the relative humidity but varies also with the changes intemperature with high sensitivity. Accordingly, it is not suitable touse the alkali metal oxide in an amount out of the above-emntionedrange. More preferably, the alkali metal oxide may be used in an amountof 0.20 to 10.00 mole percent. The alkali metal oxide may be sodiumoxide, potassium oxide or the like. Sodium oxide is particularlypreferable.

It is essential that the divanadium pentoxide content be 0.05 to 10.00mole percent. If it is less than 0.05 mole percent or in excess of 10.00mole percent, there will be no exponential functional relation nor anyother regular relation between the relative humidity and theresistivity, this rendering the sensor unusable. More preferably, thedivanadium pentoxide content may be 0.10 to 5.00 mole percent.

Furthermore it has been found that insofar as these oxide components arewithin the foregoing ranges, a certain kind of metal oxide can besubstituted for part of silicon depending upon the purpose. Thus, ZrO

and/or SiO may be substituted for 0.02 to 7.00 mole percent of siliconto impart improved chemical durability to the humidity sensor obtained.

The humidity sensor can be improved in mechanical strength bysubstituting A1 for 0.01 to 5.00 mole percent of silicon.

Substitution of CaO and/or MgO for 0.01 to 2.00 mole percent of Sifurther gives improved water resistance to the humidity sensor.

The method of making a humidity sensor in accordance with this inventionprimarily comprises steps of providing a compact of mixed powdersconsisting essentially of 0.10 to 12.00 mole percent of alkali metaloxide, 0.05 to 10.00 mole percent of divanadium pentoxide and thebalance substantially all silicon, firing the compact at about 500 C toabout 800 C and thereafter cooling the compact to room temperature.

It is necessary to conduct firing at about 500 C to 800 C. A temperaturelower than 500 C fails to impart sufficient mechanical strength to thehumidity sensor, whereas if the firing temperature is higher than 800 C,the resultant product will be vitrified to markedly increase-theresistivity thereof, rendering it in capable of exhibiting the desiredchange in resistivity. Accordingly, the firing-temperature should be asset forth above. The firing temperature may preferably be in the rangeof 600C to 750C.

The firing gives a compact exhibiting semiconductivity with satisfactorymechanical strength imparted to the humidity sensor obtained. .Thefiring time may generally be in the range of 10 to 60 minutes, morepreferably 20 to minutes.

In accordance with a preferred example of the above process, the alkalimetal oxide, divanadium pentoxide and silicon are mixed together, alongwith the previously mentioned metal oxide substituting for part of thesilicon when so desired, and a binder is then added to the mixture. Theresultant mixture is thereafter kneaded in usual manner to prepare apasty composition, which is directly molded or applied to an insulatingbase plate made of steatite porcelain, alumina porcelain or glass andequipped with electrodes. After drying, the product is placed in afurnace for firing in a surrounding atmosphere of air.

The binder permits the composition to retain its shape during moldingand imparts adhesion to the mixture when it is applied to the baseplate. The binder includes water, volatile organic solvents such aslower alcohols like methanol, ethanol, etc., aqueous solutions orvolatile organic solvent solutions of high molecular weight substancessuch as sodium carboxymethyl cellulose. The solution may contain 3 to 10wt percent of the high molecular weight substance. The binder isgenerally used in an amount of 5 to 20 wt parts per 100 wt parts of themixture.

The electrical characteristics of the humidity sensor of this inventionwill become more apparent from the examples below and drawing showingthe results obtained in the examples;

FIG. 1 is a chart bearing seven curves representing resistivity-relativehumidity data showing the beneficial the compositions of this EXAMPLE 1A base plate used has a structure as shown in FIG. 3 comprising asteatite porcelain disk 1 measuring 197 mm in diameter and 2.4 mm inthickness and about 50 micron thick gold electrodes 2 and 3, saidelectrodes 2 and 3 respectively having nine and eight comblikeprojections 2' and 3 having a width of 0.2 mm. The projections 2' and 3'are spaced apart by 0.5 mm. To 4 wt parts of each of powder compositionsNos. I to VI containing the predetermined amounts of respectivecomponents as listed in Table 1 was added 1 wt part of a bindercomprising 94.2 wt percent of diethylene glycol monobutyl ether, 5 wtpercent of ethyl cellulose and 0.8 wt percent of tannic acid, and themixture was kneaded for 24 hours by a kneader to prepare a paste, whichwas applied by screen printing to the base plate into an approximatelyl2mm-wide space between the gold electrodes 2 and 3 to a thickness ofabout 94 microns. The paste was dried at C for about 30 minutes and thenfired in the air at 680 C for 40 minutes to form a semiconductor film 4,and to the gold electrodes 2 and 3 were attached copper wires 5, 0.8 mmin diameter, whereby a humidity sensing resistive element was obtained.

TABLE 1 Proportions of Composition Nos. components (mole The sample thusprepared was then placed in a constant temperature and constant humiditychamber, wherein the relative humidity was gradually varied with thetemperature kept at 20 C to measure the changes in resistivitycorresponding to the humidity changes by a universal bridge having astandard frequency of l KHz. The results are given in FIG. 1.

The vital necessity for a minimum of 0.10 mole percent and a maximum of12.00 mole percent of sodium oxide in the resistor of this inventionwill be apparent from the data presented in FIG. 1. With the resistorscontaining 0.10 to 12.00 mole percent of sodium oxide prepared from theCompositions Nos. I to IV in accordance with this invention, theresistivity varies in exponential functional relation to the changes inthe relative humidity, this indicating that they have satisfactoryproperties as sensors for humidity measurement and control devices. Incontrast, the resistor prepared from Comparison Composition containing0.05 mole percent of sodium oxide does not exhibit an exponentialfunctional relation between the relative humidity and the resistivity.Although the sample prepared from the Comparison Composition VI with asodium oxide content of 15.00 mole percent indicates an exponentialfunctional relation between the relative humidity and the resistivity,the resistivity varies also with the temperature changes with highsensitivity. For instance, when the humidity is varied at a constanttemperature of 100 C, the sample shows the humidity-resistivitycharacteristics as represented by Curve VI, which is markedly distinctfrom Curve VI at C. Thus it is seen that the sample is not serviceablefor humidity measurement and control devices.

Approximately the same results as above will be obtained with the use ofoxides other than sodium oxide for the alkali metal oxide. However, fromthe viewpoint of accurate detection of the changes in resistivity, it ismost preferable to use sodium oxide.

EXAMPLE 2 Humidity sensors were prepared in the same manner as above byusing Compositions Nos. VII to IX of this invention and ComparisonCompositions Nos. X to XI with component proportions as given in Table2.

The humidity sensors were then tested for the determination of therelation between the relative humidity and the resistivity. The resultsare shown in FIG. 2.

The effect of divanadium pentoxide on the electrical resistivity andrelative humidity coefficient of various ceramic compositions is clearlyshown in the data represented by several curves of FIG. 2. With thesamples of this invention prepared from Compositions VII to IXcontaining 0.05 to [0.00 mole percent of divanadium pentoxide, theresistivity varies in ex ponential functional relation to the changes inthe relative humidity, whereas in the case of the samples prepared fromthe Comparison Compositions X and XI containing 0.03 mole percent and15.00 mole percent respectively of divanadium pentoxide, there is foundno exponential functional relation nor any other regular relationbetween the relative humidity and the resistivity. The samples aretherefore unusable for humidity measurement and control devices.

EXAMPL To 4 wt parts of a composition consisting of 94.10

mole percent of Si, 2.50 mole percent of Na O, 1.00

percent of diethylene glycol monobutyl ether, 5 wt percent of ethylcellulose and 0.8 wt percent of tannic acid, and the mixture was kneadedby a kneader for 24 hours to prepare a pasty composition. Thecomposition was applied to a base plate as shown in FIG. 3 to prepare ahumidity sensor of this invention in the same manner as in Example 1.

The humidity-resistivity characteristics of this humidity sensor weremeasured in the same manner as in Example 1 with the results shown inFIG. 4. It is apparent that the sensor is capable of accuratelyindicating the changes in resistivity overthe entire range of relativehumidity.

Although the invention has been described with respect to certainspecific examples, it will be appreciated that many modifications andchanges may be made by those skilled in the art without departing fromthe spirit of the invention.

What we claim is:

1. A humidity sensor having a negative relative humidity coefficient ofresistivity which comprises 0.10 to 12.00 mole percent of alkali metaloxide, 0.05 to 10.00

mole percent of divanadium pentoxide and the balance substantially allsilicon.

2. The humidity sensor as set forth in claim 1 wherein said alkali metaloxide is sodium oxide.

3. The humidity sensor as set forth in claim 1 wherein said alkali metaloxide is contained in the amount of 0.20 to 10.00 mole percent.

4. The humidity sensor as set forth in claim 1 wherein said divanadiumpentoxide is contained in the amount of 0. 10 to 5.00 mole percent.

5. The humidity sensor as set forth in claim 1 wherein at least one ofZrO and Si0 is substituted for 0.02 to 7.00 mole percent of saidsilicon.

6. The humidity sensor as set forth in claim 1 wherein A1 0 issubstituted for 0.01 to 5.00 mole percent of said silicon.

7. The humidity sensor as set forth in claim 1 wherein at least one ofCaO and MgO is substituted for 0.01 to 2.00 mole percent of saidsilicon.

8. A method of producing a humidity sensor having a negative relativehumidity coefficient of resistivity, which comprises the steps ofproviding a compact, of mixed powders consisting essentially of 0.10 to12.00 mole percent of alkali metal oxide, 0.05 to 10.00 mole percent ofdivanadium pentoxide and the balance substantially all silicon, firingsaid compact at 500 C to 800 C and thereafter cooling the tired compactto room temperature.

9. The method as set forth in claim 8 wherein said alkali metal oxide issodium oxide.

10. The method as set forth in claim 8 wherein said firing temperatureis in the range of 600C to 750C.

' STATES '1 A N' E @FEFEQE 'Cfiii ii's i March 20, 1973' Patent No.

lnventofls) Kiyoshi Sumi and Osamu Asakura It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

In the heading insert v Claims priority of Japanese Application SerialNo; 73579, filed August 21, l970.-

Signed and sealed this lLl th day of August 1973 (SEAL) Attest:

EDWARD M. FLETCDER R RENE D. TEGTD LEIYER Acting Commissioner of PatentsAttesting Officer FORM I O-1050 (10-69) uscoMM-Dc 60375-P69 U.$.GOVERNMENT PRINTING OFFICE: I969 O-'355-334

2. The humidity sensor as set forth in claim 1 wherein said alkali metaloxide is sodium oxide.
 3. The humidity sensor as set forth in claim 1wherein said alkali metal oxide is contained in the amount of 0.20 to10.00 mole percent.
 4. The humidity sensor as set forth in claim 1wherein said divanadium pentoxide is contained in the amount of 0.10 to5.00 mole percent.
 5. The humidity sensor as set forth in claim 1wherein at least one of ZrO2 and SiO2 is substituted for 0.02 to 7.00mole percent of said silicon.
 6. The humidity sensor as set forth inclaim 1 wherein Al2O3 is substituted for 0.01 to 5.00 mole percent ofsaid silicon.
 7. The humidity sensor as set forth in claim 1 wherein atleast one of CaO and MgO is substituted for 0.01 to 2.00 mole percent ofsaid silicon.
 8. A method of producing a humidity sensor having anegative relative humidity coefficient of resistivity, which comprisesthe steps of providing a compact of mixed powders consisting essentiallyof 0.10 to 12.00 mole percent of alkali metal oxide, 0.05 to 10.00 molepercent of divanadium pentoxide and the balance substantially allsilicon, firing said compact at 500* C to 800* C and thereafter coolingthe fired compact to room temperature.
 9. The method as set forth inclaim 8 wherein said alkali metal oxide is sodium oxide.
 10. The methodas set forth in claim 8 wherein said firing temperature is in the rangeof 600*C to 750*C.